1. Field of the Invention
The present invention relates to a green soybean whose green color is prevented from discoloring and a cooked and frozen product thereof. More particularly, the present invention relates to a deep-green soybean in which a green color is retained even in a condition exposed to illumination of light. Even more particularly, the present invention relates to a green soybean which contains chlorophyll a of not less than 3.8 g/100 g wet weight and/or a material accumulated in tissues for suppressing decomposition of chlorophyll, as well as to a green soybean in which an anti-oxidation material, e.g., β-carotene, is contained especially in a pod in a more amount. Further, the present invention relates to a product distributable under the chilled condition of these green soybeans.
2. Description of the Related Art
Recently, cooked green soybeans have been distributed in markets in the form of frozen or thawed products, and have been produced in an increasing amount because of convenience in handling. However, chlorophyll contained as a green color component in green soybeans is unstable for various reasons and tends to easily decompose by external factors. Considering the stability of chlorophyll from the viewpoint of temperature zones during the distribution, when products of frozen green soybeans are held at a temperature of −20° C. in a dark place, the problem of discoloration of the green basically does not arise, but when the products are exposed to illumination of light, there arises the problem of discoloration of the green. On the other hand, when frozen products of cooked green vegetables are thawed and preserved in a chilled temperature zone lower than the normal temperature under illumination of light, a vegetable's green color fades hour by hour and day by day. Such discoloration gives rise to a serious problem in processes of production and distribution because a commodity value of cooked green vegetables lowers noticeably with the discoloration. As hitherto known in the art, a major factor of discoloring the green is light and it is very difficult to retain the green, particularly, in the chilled temperature zone. Although the green can be retained to some extent by shielding off light, this solution is disadvantageous in that the condition of the products cannot be confirmed by looking at the inside of the package and consumers lose one of the ways of checking product quality. When the products are lay out for sale at shop without shielding off light, they are placed under illumination of light all day and therefore the discoloration of green is serious.
It is known that retention of a vegetable's green color is greatly affected by temperature and moisture (see “Handbook of Techniques for Packing Functional Foods”). For the purpose of retaining and protecting the green in a frozen temperature zone, a vacuum packing method or a gas replacing method has been developed (see Japanese Unexamined Patent Application Publication No. 56-1846). When green vegetables are preserved in a dark place under a frozen condition of lower than −20° C., the reaction temperature is low and hence discoloration of the vegetable's green does not cause a problem in practice. When green vegetables are held in showcases, etc. under illumination of light or at temperatures of higher than −20° C., the effect of retaining the green color is confirmed with such a method.
For the purpose of preventing discoloration of cooked vegetables especially in the chilled temperature zone in which the discoloration occurs noticeably, a method using a protamine solution (Japanese Unexamined Patent Application Publication No. 6-7083), a method of adding kojic acid, etc. (Japanese Unexamined Patent Application Publication No. 8-56610), and a method using calcium salts have been invented in the past. However, the effect of any of those methods is not sufficient and further development of the technology is demanded. In addition, a vacuum packing method or a gas replacement packing method has been developed (Japanese Unexamined Patent Application Publication No. 10-304820). Although the effect of that method is relatively significant, that method requires special equipment and gives rise to a problem of cost efficiency.